The present invention relates generally to blow out preventers for use in preventing blow out of well pressure during oil well and other drilling operations. More particularly, the present invention relates to ram type blow out preventers which are commonly used to prevent blow out of well pressures in relatively high pressure wells.
Ram type blow out preventers basically include a resilient ram seal, a ram block for biasing the ram seal against the drill string to provide desired sealing, and a ram block holder for mounting the ram block and ram seal within the blow out preventer assembly. Ram seals have been conventionally made from elastomeric materials such as rubber. The ram seal usually includes two ram seal elements which are semi-circular elastomer bodies having a diametrical portion and a circumferential portion. The diametrical portion of each seal element generally includes a central collar section. The two seal elements are typically mounted within the ram block so that the central collar sections are aligned to provide a central orifice through which the drill string is passed.
Many times it is desirable to change the diameter of the drill string during drilling operations. Attempts have been made to design a ram assembly which is capable of accommodating different sized drill strings. Such a variable diameter ram seal assembly is especially desirable since it would avoid the necessity of having to change the ram seal assembly every time a change in drill string diameter is made.
The ram type blow out preventer disclosed in U.S. Pat. No. 3,897,040 is an example of an attempt to provide a variable inside diameter ram seal. This seal assembly is designed to be normally in a full-open bore position to accommodate the larger drill string diameters with means being provided to force the seal element inward to provide sealing of smaller drill strings. Although this particular variable diameter ram seal assembly may be well suited for its intended purpose, there is still a continuing need to provide improved ram type seal assemblies which are capable of providing positive, leak-free sealing of variable diameter drill strings at relatively high pressure of 5,000 p.s.i. and above.